


On the Rocks

by thewinchesterlifestyle



Category: Supernatural
Genre: Age Difference, Anal Gaping, Barebacking, Beach Sex, Bottom Dean, Breeding, Butt Plugs, Come Inflation, Fertile Dean, Fingering, Forced Pregnancy, Large Cock, M/M, Merman Castiel, Mildly Dubious Consent, Mpreg, Multiple Orgasms, Ocean Sex, Older Castiel, Painful Sex, Pheromones, Pregnant Dean, Rough Sex, Size Difference, Size Kink, Teen Dean, Top Castiel, carrier dean, castiel/other(s) - Freeform
Language: English
Status: Completed
Published: 2017-06-11
Updated: 2017-06-11
Packaged: 2018-11-13 00:03:22
Rating: Not Rated
Warnings: Underage
Chapters: 1
Words: 2,397
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11172801
Author URL: https://archiveofourown.org/users/thewinchesterlifestyle/pseuds/thewinchesterlifestyle
Summary: Castiel planned on using this visit to Dean’s shore to make sure the young human was thoroughly bred and swollen with his release before he left.





	On the Rocks

**Author's Note:**

> Written for a prompt.
> 
> I'm on a roll today!!

Castiel sucked a mark onto the young, pretty green-eyed human’s neck. He could taste the salt of the ocean on Dean’s skin and it mixed with the usual flavors he’d found when he’d first found Dean weeks ago. Small, soft and eager as he swam in the shallows.

This wasn’t the first human he’d found, wasn’t the first he’d vigorously fucked and planned to breed, but Dean was small, soft and released the most arousing sounds when Castiel was pounding into him. The majority of the humans he'd found, lured in with his pheromones, were further along in age and had plenty of breeding seasons behind them. Not Dean. Dean was young, fresh and clearly eager to be bred.

Castiel had found that those who had just come in season were often the best to fuck and breed, not only for the fertility but also for the blissful pleasure that could be found in their bodies. He was very pleased with finding this particular human. Dean had only just come into season to be bred and despite his youth was _very_ fertile. He didn't know much about humans and their mating habits but he knew one so inexperienced was often far more pliant and often desperate for their first breeding.

“Please Cas. Please.” Dean’s voice was a low and wrecked tone. He could feel the young human, barely in season by a few weeks if he had to guess, spreading his legs further where Castiel rested between them. Dean’s warm body rubbed against Castiel’s sheath, coaxing his very long and thick cock out, while he mouthed along the underside of Dean’s jaw. “I’m ready. I made sure.”

Humans were not meant to take a cock the size and girth of a merman’s, his cock was over sixteen inches long with an increasing thickness towards the base that was designed to breed with a mermaid or a receptive merman. Humans were far smaller but that didn’t stop his kind from fucking and breeding humans. It was effortless to enthrall one with a mere glimpse or a soft song not unlike a siren.

They would come back to the ocean over and over again after they’d been fucked for the first time. Dean had been no different. And once a human was bred they would find themselves compelled to return to the ocean to birth, offspring slipping into the ocean and the human even more enthralled than before. It helped to ensure they would return for future breedings.

He started to angle himself in order to push inside Dean, the small human had taken to loosening himself to take Castiel’s large cock, but something hard was in the way. Castiel released a low snarl and reached down to find a hard object buried inside Dean’s entrance. He frowned and pulled on it until the object popped free. “What’s this?” Castiel had never seen one before and he almost flung it across the rocks.

“A butt plug.” Dean replied before he was trying to push himself down onto Castiel’s cock but Castiel paused him.

“What is its purpose?”

He watched Dean blinking at him and felt how the human slumped against him where he was pressing Dean against a smooth stone. “Ummmm…well it can hold my hole open so I don’t have to worry about prep or it can keep your come…inside of me. You’re always talking about wanting to…. breed me full.” there was a flush on Dean's freckled cheeks as he mentioned the last part of the sentence.

Castiel’s eyes hooded at the thought of Dean walking around on land with his seed locked safely inside and his small belly round with it. The thought had a possessive part of him practically humming with satisfaction. The very notion of being able to breed Dean and ensure the human caught with his offspring had him tightening his hold.

Over the years Castiel had fucked and bred plenty of humans and other mermen and mermaids but this was the first time he’d come across a human barely in season and receptive to him who had eagerly offered his untouched hole for Castiel’s enjoyment. Most had been in season for awhile and required more pheromones to tempt into breeding. Dean's body was slick and eager with only a hint of pheromones to coax Dean into spreading his legs wider.

He shoved the plug into the pouch where he kept other important items for hunts and while he traveled. It would be going back inside Dean after he was finished breeding with the human, he could smell Dean’s new fertility that came with a human mature enough to carry offspring, and the plug would only be going back in after he had pumped as many loads of his come needed to breed Dean into the young human’s hole. 

Castiel planned on using this visit to Dean’s shore to make sure the young human was thoroughly bred and swollen with his release before he left. It was fortunate that Dean was giving off a subtle scent of fertility so Castiel knew Dean could be bred successfully. He briefly wondered if humans went into season far earlier than Castiel's own kind because they needed to make use of more seasons to breed and populate or if the first few fertile seasons were ignored until the fertile human had grown. It didn't matter. Dean was fertile immediately upon maturity and Castiel was going to breed him regardless.

After today he wouldn’t be back for a while, mating season was beginning in his village as well as the surrounding villages and if he wanted to breed a variety of mermaids and receptive mermen he would need to leave shore for the entirety of the season. Mermaids always came in season first and receptive merman once the mermaids were all bred. By the time he returned Dean would be beautifully swollen, belly huge and round, with Castiel’s growing offspring.

It didn’t take much to shift them both so the bulbous head of his wide cock was pressing against Dean’s loosened hole and once he’d gotten a good grip on Dean he pushed up, shoving inside and groaning at the sensation of slowly starting to inch inside. The young human’s mouth fell open, soft cheeks flushed and eyes dazed, as Castiel started to sheath the full length of his impressive cock into the small body in his hold.

Inside Dean was gloriously tight and hot, muscles contracting against Castiel’s cock as it spread him wide, while Castiel pressed further in. It was harder to mate at such an angle and against the rocks but Castiel couldn’t take Dean underwater to breed him as he typically would with another mermaid or merman.

Humans had to be fucked and bred against the rocks or in shallow water so Castiel wouldn’t suffer discomfort.

In front of him Dean was moaning, eyes pained as they always were at the very beginning, and the young human’s legs had spread wide so Castiel could get as deep as possible into him. He tightened his hold and pushed past a bit of resistance he found the deeper he pressed, ignoring the wrecked little cry it earned, as more of his cock sank inside. The base of his cock was near five inches wide and very hard to fuck inside such a small hole not equipped to take it like his kind could but it wouldn't stop him from pushing as deep as possible.

As Dean shuddered, breathing ragged and uneven, Castiel started to thrust up into Dean. Each drag of his cock against Dean’s warm insides had his breathing picking up and warmth building inside him. Fucking and breeding a human was one of his favorite things and the sight of his cock against the think skin of human’s flat belly was breathtaking. He could see how full he'd fucked the small human and it had a smug sense of satisfaction filling him.

“Fuck, Cas.” Dean’s voice was rough and his young face was screwed up in pained pleasure, “You’re so  _thick_.” This would be the first time that Castiel fucked Dean with the intention to breed, the other times he’d not pushed nearly his full length inside, and if Dean had been with other humans the full size of Castiel would be overwhelming. From what Castiel had seen of human males their cocks were not as thick, long or ridged as Castiel’s species.

He constantly wondered how they accomplished anything with them.

Dean’s own cock was small, so much smaller than Castiel’s, as it rested between them neglected and soft. It didn’t matter, the receiving breeding partner was meant to take pleasure from being bred. He was glad he'd gotten Dean slowly used to his size, letting only a hint of his pheromones saturate the human's senses, because it would be far harder to have gotten to this point without possibly hurting Dean.

As Castiel started to fuck in earnest, slamming up into Dean’s pliant body, Dean’s hands gripped at him as the human Castiel planned to breed moaned loudly. The clench of Dean’s body around him had Castiel moving forward quicker, as best he could in such a small and tight hole, as he fucked towards his first release.

Castiel resolved to breed with Dean in the shallows next time. It would be far easier if the human rode his cock and Castiel didn’t have to worry about seriously damaging Dean’s delicate skin on sharp rocks. It would be horrible if they attracted a mershark because Castiel wasn’t equipped to defend himself and the mershark would no doubt choose to breed Dean or simply eat the human.

He kept thrusting forward and working himself into Dean. Castiel neared his orgasm and watched how one of Dean’s hands moved from him to fly over his cock, a useless and pointless endeavor in his mind. Castiel watched how Dean’s mouth dropped open and his pretty human’s hole tightened around him as he came.

Dean’s moan seemed to ride the surf.

At least it had caused him a delicious jolt of pleasure. Castiel kept almost viciously fucking forward even as Dean went pliant until Castiel tensed, body snapping tight, as his cock emptied his load into Dean’s hole. He imagined Dean swollen with his offspring, belly bulging and overfilled. Instinctively Dean would be driven to the water when it was time to birth and Castiel would happily take his offspring back to his village while Dean could recover on land, hopefully in anticipation of being bred again.

Castiel currently had no intention of letting anyone else breed Dean in the near future but he knew circumstances changed and sometimes he only bred one offspring per human.

He moved forward to bite against where Dean’s neck met his shoulder as his cock emptied another load into Dean, his release hopefully to fill Dean’s womb and impregnate the human. His sharp teeth easily cut through the skin to leave a kind of claim that others would know this human had been bred by one of his kind.

Under him Dean cried out at a shudder pain radiating from where Castiel had bitten into soft, easily shredded skin.

Castiel cooed at him lowly in his throat while rubbing soothing circles against Dean’s warm skin. He dragged his tongue over the wound and where blood had trailed down Dean’s freckled skin. The taste was rich and warm as he bared his sharp teeth in pleasure. “Mine.” He breathed possessively before nuzzling against Dean. A hand rested against Dean’s stomach where a small bulge had formed from the amount of come he’d already pumped into Dean. He could feel his own cock nestled inside Dean’s body and groaned in satisfaction when Dean clenched around him with a low moan.

They stayed like that for several minutes until Castiel reluctantly pulled out of Dean’s warm body to roll the human over onto his belly. Dean’s once tiny hole gaped obscenely, leaking copious amounts of Castiel’s come and a little blood. He traced his fingers over the entrance, memorized that _this_ was how humans bred. It wouldn’t take him long to recover and soon Castiel was gliding his stiff cock back into Dean’s hole.

It slipped inside, splitting the human’s plump cheeks apart, until he was pressed all the way up against Dean’s ass. Beneath him the human whined lowly, body completely pliant and loose on the rocks, as Castiel began fucking into Dean’s hole once more.

Castiel dedicated hours to breeding Dean, praising the human as his belly swelled obscenely with Castiel’s come, until Castiel caught the tiniest hint that Dean had caught. His senses far superior and honed to pick up a successful breeding. He continued fucking into Dean, long cock pounding into Dean’s spent and fucked out body, until he came for the last time with a groan of appreciation.

There was nothing like a successful breeding and Castiel felt smug satisfaction filling him. Dean only laid there on the rock, cheeks flushed and eyes hooded with a dazed look to them. His mouth was slack as he panted lowly. Castiel had clearly fucked the young human into exhaustion but it was with great success.

He eased out of Dean’s fucked open hole and upon seeing some of his release escaping Castiel reached for the plug to push it back in. Dean’s face twitched but he remained relaxed and wore a sated expression that had Castiel smiling, pleased.

The young human’s belly was swollen with Castiel’s release and it was more than obvious Castiel had fucked him full over the hours he’d spent near the rocks just off the beach.

Castiel’s cock disappeared once more into his sheath, the act of breeding complete, as he sunk deeper into the water and sighed at the feeling of the cool ocean against his warm body. Dean blinked, dazed and spent, as Castiel gazed one last time at his most recently bred human.

“Goodbye, Dean.”

Dean’s head turned to look at him and Castiel’s lips quirked at the young human’s slack mouth. Green eyes blinked again and Dean whined lowly as he shifted, “Bye, Cas.” He managed.

Castiel disappeared back into the ocean, satisfied at a successful breeding, as he swam towards his village with a content smile on his face. He couldn’t wait to see the offspring Dean gave him.

**Author's Note:**

> Thought you all could use some older, experienced Merman!Cas thoroughly enjoying himself while breeding Dean. How did you like my reasoning behind how and why Castiel's species has such a HUGE cock? I couldn't resist.
> 
>  
> 
> I know a bunch of you love these kinds of fics. I'm excited to see what you all thought! Don't be shy! This is a safe place and you can always comment anonymously!


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